Stronger articles can be made by blow molding if the plastic material is bi-axially oriented. This requires stretching the material in different directions while the material is at its "orientation temperature" which is the temperature at which crystallization begins.
In order to obtain some cooling of the parison in the injection mold, the temperature of the molten plastic is reduced by leaving the parisons in the injection mold while they are cooled by inside cooling through the core rods and outside cooling by contact with cooled walls of the injection mold cavities.
Further control of the temperature is obtained by indexing the core rods to a temperature control station at which the core rods are located in chambers through which air circulates. In the preferred construction, the air circulation chambers are cylindrical with clearance around all sides of the core rods, and air is introduced at a center region and discharges from both ends of the chambers. In order to prevent flow by gravity of the parison material toward the bottom sides of the core rods while the material is soft, the core rods are rotated while in the temperature-controlling chambers; and this rotation also equalizes the cooling effect of the air which is introduced into the cooling chambers.
From a temperature control station, the parisons may be indexed to a pre-blow station where the parison is stretched circumferentially and axially by a initial blowing operation; or the parison can be transferred directly to the final blow station where it is blown to the final dimensions of the article being made on the machine. From the blow mold, the blown articles are transferred to a stripper station at which they are removed from the core rods and the core rods are indexed to the injection station to receive a new set of parisons. The blowing, stripping and indexing back to the injection station are conventional and well-understood in the blowing machine art.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.